Stink bugs and other hemipteran insects (heteroptera) are an important agricultural pest complex. Worldwide, over 50 closely related species of stink bugs are known to cause crop damage. McPherson & McPherson (2000) Stink bugs of economic importance in America north of Mexico, CRC Press. Hemipteran insects are present in a large number of important crops including maize, soybean, fruit, vegetables, and cereals.
Stink bugs go through multiple nymph stages before reaching the adult stage. These insects develop from eggs to adults in about 30-40 days. Both nymphs and adults feed on sap from soft tissues into which they also inject digestive enzymes causing extra-oral tissue digestion and necrosis. Digested plant material and nutrients are then ingested. Depletion of water and nutrients from the plant vascular system results in plant tissue damage. Damage to developing grain and seeds is the most significant as yield and germination are significantly reduced. Multiple generations occur in warm climates resulting in significant insect pressure. Current management of stink bugs relies on insecticide treatment on an individual field basis. Therefore, alternative management strategies are urgently needed to minimize ongoing crop losses.
RNA interference (RNAi) is a process utilizing endogenous cellular pathways, whereby an interfering RNA (iRNA) molecule (e.g., a double stranded RNA (dsRNA) molecule) that is specific for all, or any portion of adequate size, of a target gene results in the degradation of the mRNA encoded thereby. In recent years, RNAi has been used to perform gene “knockdown” in a number of species and experimental systems; for example, Caenorhabditis elegans, plants, insect embryos, and cells in tissue culture. See, e.g., Fire et al. (1998) Nature 391:806-11; Martinez et al. (2002) Cell 110:563-74; McManus and Sharp (2002) Nature Rev. Genetics 3:737-47.
RNAi accomplishes degradation of mRNA through an endogenous pathway including the DICER protein complex. DICER cleaves long dsRNA molecules into short fragments of approximately 20 nucleotides, termed small interfering RNA (siRNA). The siRNA is unwound into two single-stranded RNAs: the passenger strand and the guide strand. The passenger strand is degraded, and the guide strand is incorporated into the RNA-induced silencing complex (RISC). Micro inhibitory ribonucleic acids (miRNAs) are structurally very similar molecules that are cleaved from precursor molecules containing a polynucleotide “loop” connecting the hybridized passenger and guide strands, and they may be similarly incorporated into RISC. Post-transcriptional gene silencing occurs when the guide strand binds specifically to a complementary mRNA molecule and induces cleavage by Argonaute, the catalytic component of the RISC complex. This process is known to spread systemically throughout some eukaryotic organisms despite initially limited concentrations of siRNA and/or miRNA such as plants, nematodes, and some insects.
Only transcripts complementary to the siRNA and/or miRNA are cleaved and degraded, and thus the knock-down of mRNA expression is sequence-specific. In plants, several functional groups of DICER genes exist. The gene silencing effect of RNAi persists for days and, under experimental conditions, can lead to a decline in abundance of the targeted transcript of 90% or more, with consequent reduction in levels of the corresponding protein. In insects, there are at least two DICER genes, where DICER1 facilitates miRNA-directed degradation by Argonaute1. Lee et al. (2004) Cell 117(1):69-81. DICER2 facilitates siRNA-directed degradation by Argonaute2.
The overwhelming majority of sequences complementary to insect DNAs (such as, for example, the 9,000+ sequences identified in U.S. Pat. No. 7,612,194 and U.S. Patent Publication Nos. 2007/0050860, 2010/0192265, and 2011/0154545) do not provide a plant protective effect when used as dsRNA or siRNA. For example, Baum et al. (2007) Nature Biotechnology 25:1322-1326, describe the effects of inhibiting several WCR gene targets by RNAi. These authors reported that 8 of the 26 target genes they tested were not able to provide experimentally significant coleopteran pest mortality at a very high iRNA (e.g., dsRNA) concentration of more than 520 ng/cm2.
The authors of U.S. Pat. No. 7,612,194 and U.S. Patent Publication No. 2007/0050860 made the first report of in planta RNAi in corn plants targeting the western corn rootworm. Baum et al. (2007) Nat. Biotechnol. 25(11):1322-6. These authors describe a high-throughput in vivo dietary RNAi system to screen potential target genes for developing transgenic RNAi maize. Of an initial gene pool of 290 targets, only 14 exhibited larval control potential. One of the most effective double-stranded RNAs (dsRNA) targeted a gene encoding vacuolar ATPase subunit A (V-ATPase), resulting in a rapid suppression of corresponding endogenous mRNA and triggering a specific RNAi response with low concentrations of dsRNA. Thus, these authors documented for the first time the potential for in planta RNAi as a possible pest management tool, while simultaneously demonstrating that effective targets could not be accurately identified a priori, even from a relatively small set of candidate genes.
Another potential application of RNAi for insect control involves parental RNAi (pRNAi). First described in Caenorhabditis elegans, pRNAi was identified by injection of dsRNA into the body cavity (or application of dsRNA via ingestion), causing gene inactivity in offspring embryos. Fire et al. (1998), supra; Timmons and Fire (1998) Nature 395(6705):854. A similar process was described in the model coleopteran, Tribolium castaneum, whereby female pupae injected with dsRNA corresponding to three unique genes that control segmentation during embryonic development resulted in knock down of zygotic genes in offspring embryos. Bucher et al. (2002) Curr. Biol. 12(3):R85-6. Nearly all of the offspring larvae in this study displayed gene-specific phenotypes one week after injection. Although injection of dsRNA for functional genomics studies has been successful in a variety of insects, uptake of dsRNA from the gut environment through oral exposure to dsRNA and subsequent down-regulation of essential genes is required in order for RNAi to be effective as a pest management tool. Auer and Frederick (2009) Trends Biotechnol. 27(11):644-51.
Parental RNAi has been used to describe the function of embryonic genes in a number of insect species, including the springtail, Orchesella cincta (Konopova and Akam (2014) Evodevo 5(1):2); the brown plant hopper, Nilaparvata higens; the sawfly, Athalia rosae (Yoshiyama et al. (2013) J. Insect Physiol. 59(4):400-7); the German cockroach, Mattel/a germanica (Piulachs et al. (2010) Insect Biochem. Mol. Biol. 40:468-75); and the pea aphid, Acyrthosiphon pisum (Mao et al. (2013) Arch Insect Biochem Physiol 84(4):209-21). The pRNAi response in all these instances was achieved by injection of dsRNA into the hemocoel of the parental female.